An antibody fusion protein linking a protein of interest to an immunoglobulin constant region possesses both the biological activity of the linked protein as well as the advantages associated with the presence of the immunoglobulin moiety. The creation of such fusion proteins helps to ensure the efficient production and secretion of proteins of interest. Furthermore, these fusion proteins often exhibit novel properties such as increased circulating half-life that are of significant therapeutic advantage.
In adult mammals, FcRn, also known as the neonatal Fc receptor, plays a key role in maintaining serum antibody levels by acting as a protective receptor that binds and salvages antibodies of the IgG isotype from degradation. IgG molecules are endocytosed by endothelial cells, and if they bind to FcRn, are recycled out into circulation. In contrast, IgG molecules that do not bind to FcRn enter the cells and are targeted to the lysosomal pathway where they are degraded. A variant IgG1 in which His435 is mutated to alanine results in the selective loss of FcRn binding and a significantly reduced serum half-life (Firan et al. 2001, International Immunology 13:993).
The Fc portion of an IgG molecules includes two identical polypeptide chains, with each polypeptide chain engaging a single FcRn molecule through its FcRn binding site (Martin et al., 1999, Biochemistry 38:12639). Earlier studies indicate that each Fc portion requires both FcRn binding sites for serum persistence. A heterodimeric Fc fragment containing a wild-type FcRn-binding polypeptide chain and a mutated non-FcRn-binding polypeptide chain has a significantly reduced serum half-life compared to a homodimeric wild-type Fc fragment (Kim et al., 1994, Scand. J. Immunol. 40:457-465). Such a heterodimeric Fc fragment containing only one FcRn binding site is recycled less efficiently than a homodimeric wild-type Fc fragment and is preferentially trafficked to lysosomes for degradation (Tesar et al., 2006, Traffic 7:1127). These observations have direct relevance to the effective application of therapeutics involving FcRn binding partners including IgG antibodies or their Fc portions. For example, U.S. Pat. No. 7,348,004 describes a fusion protein that specifically requires intact FcRn binding or even enhanced FcRn binding for its improved biological properties such as increased serum half-life and enhanced bioavailability.
More recent studies have identified the expression of FcRn in phagocytes and suggested a novel role of FcRn in IgG-mediated phagocytosis (Vidarsson et al. 2006, Blood 108:3573). IgG-opsonized pathogens are internalized into phagosomes by FcRn, providing an effective means to mark pathogens for ingestion and destruction by phagocytes. The IgG1 variant with a H435A mutation exhibits significantly reduced opsonization activity.